Two major U.S. airlines were involved in the September 11, 2001, terrorist attacks: American Airlines and United Airlines. Four specific flights were hijacked that morning: American Airlines Flight 11 (crashed into the North Tower of the World Trade Center), United Airlines Flight 175 (crashed into the South Tower of the World Trade Center), American Airlines Flight 77 (crashed into the Pentagon), and United Airlines Flight 93 (crashed in a field in Somerset County, Pennsylvania, after passengers and crew fought back against the hijackers). These tragic events led to an immediate and permanent transformation of global aviation security, resulting in the creation of the Transportation Security Administration (TSA) in the United States and the implementation of reinforced cockpit doors and stricter passenger screening protocols worldwide. In the decades since, both airlines have retired these specific flight numbers out of respect for the victims and their families.